A general multiple-optical-axis photoelectric sensor includes a light projecting unit in which a plurality of light projecting elements are arranged in line and a light receiving unit in which light receiving elements as many as the light projecting elements are arranged in line. The light projecting elements and the light receiving elements are arranged so as to face to each other on a one-on-one basis, and a detection area composed of a plurality of optical axes is set.
The light projecting unit causes the light projecting elements to sequentially emit the light beam. The light receiving unit takes out the amount of light that each light receiving element receives from a corresponding light projecting element in synchronization with a light emitting operation of the light projecting element. Therefore, a light blocking state is sequentially detected in each optical axis of the multiple-optical-axis photoelectric sensor. The light receiving unit determines whether an object exists in a detection area using a detection result of each optical axis, and outputs a signal indicating a determination result. The light projecting unit and the light receiving unit are connected through a communication line in order to synchronize the light projecting unit and the light receiving unit with each other. Alternatively, the light projecting unit and the light receiving unit are synchronized with each other by optical communication between the light projecting unit and the light receiving unit.
For example, the multiple-optical-axis photoelectric sensor is installed as a device securing safety of a worker in a production site. For example, operation of a production facility is stopped when the light blocking state is detected in one of the optical axes in the detection area of the multiple-optical-axis photoelectric sensor. Depending on a production apparatus, it is necessary to provide the detection area of the multiple-optical-axis photoelectric sensor in a passage through which a pre-processing or post-processing workpiece is conveyed. However, productivity is degraded in the case that the workpiece blocks the optical axis to stop the production facility.
A muting function of temporarily disabling a safety function of the multiple-optical-axis photoelectric sensor exists as a function of establishing the safety and the productivity in the production site. Basically, the muting function disables the whole detection area based on two independent muting signals output from a sensor or a switch. The whole detection area is disabled only in a period during which the muting signal is output from the sensor or switch according to a correct sequence or maintained (for example, see Japanese Patent No. 5229310).
Because the muting signal is used as a trigger disabling the safety function of the multiple-optical-axis photoelectric sensor, the sequence (specifically, output order and output time of two muting signals) of the muting signal is strictly checked. When the sequence deviates from a previously-defined condition, generally a determination of a danger state is made to cut off power supplied to the production facility.
In the muting function, for example, a proximity switch or a limit switch is used as the sensor or switch that outputs the muting signal. The sensor or the switch is installed on the passage through which the workpiece is conveyed, and the sensor or the switch detects the workpiece to output the muting signal. The sequence in the muting function is defined based on a position where the sensor or switch is installed, a shape of the workpiece, and a conveying speed of the workpiece. Therefore, in the case that the deviation is generated at the position where the sensor or switch is installed, the muting signal is not output according to the predetermined sequence. As a result, the productivity is disturbed because the production facility is stopped. A phenomenon in which, although the workpiece of the muting target passes, the muting function is abnormally operated to stop the production facility is called a “muting error”. The muting error is possibly generated by the shape of the workpiece and a vibration of the passage during the conveyance of the workpiece in addition to the installation state of the sensor.
As to a countermeasure suppressing the muting error, in the conventional multiple-optical-axis photoelectric sensor, after the multiple-optical-axis photoelectric sensor and a muting sensor are installed, a test run in which the workpiece is conveyed on a trial basis is repeated, and the installation states of the sequence and the muting sensor are narrowed in order to normally operate the muting function. For this reason, a large amount of man-hour is required during start-up of the production facility. Additionally, every time the workpiece is switched (exchange of arrangement) in the production facility, it is necessary to narrow the installation states of the sequence and the muting sensor in order to meet the shape and conveying speed of the post-switch workpiece, which results in a problem in that the man-hour becomes huge.
In order to solve the problem, Japanese Patent No. 5229310 discloses a multiple-optical-axis photoelectric sensor having a function of dividing the sequence into a plurality of stages to monitor the sequence and making a notification regarding in which stage the muting error is generated in order to be restored from the muting error. However, although this technique contributes to the shortening of the restoration time after the muting error is generated, the technique insufficiently exerts the effect with respect to the suppression of the generation of the muting error and the shortening of the start-up time of the production facility.